The present invention relates to a process for the manufacture of vinyl chloride and hydrogen chloride from 1,2-dichloroethane by thermal cracking of the 1,2-dichloroethane and subsequent cooling and rectification of the product mixture.
The thermal cracking of 1,2-dichloroethane is performed according to known processes in which the 1,2-dichloroethane is subjected to indirect heating in a reaction furnace and split into vinyl chloride and hydrogen chloride at temperatures ranging from 480.degree. to 560.degree. C. Thermal cracking is not complete, but yields a product mixture which contains numerous by-products of different chemical composition apart from 1,2-dichloroethane and the main products referenced above. Among said by-products, saturated and unsaturated aliphatic compounds, aromatics and carbon black may be cited. Deposits of carbon black and coke in the reaction furnace require a shutdown of the furnace at intervals of some months for decoking operations. The formation of by-products is partly attributable to the fact that attempts to produce absolutely pure dichloroethane at economically justified costs, have so far been unsuccessful. Another cause is that the reaction products are thermally unstable at the required high temperature and undergo decomposition to carbon in a series of further reactions.
As described in patents on this subject, the temperature of the hot reaction gases can subsequently be lowered indirectly with the aid of a cooling fluid and directly with the aid of precooled reaction product.
In practice, however, the method of indirect cooling has proved not to be advantageous because undesirable and troublesome coke deposits were encountered within a short time.
In a cooling system of conventional design for lowering the temperature of the hot reaction gases from 540.degree. C. to 200.degree. C., an undesirable pressure drop will be encountered after a few weeks of service time owing to increasing coke deposits in the tubes.
Therefore, continued efforts have been made to end as quickly as possible the thermal instability of the hot gas mixture leaving the reaction furnace. This has been achieved through direct cooling by continuous injection of cold liquid dichloroethane into the hot gas stream. By this method, the reaction gases were quenched to a temperature at approximately 130.degree. to 60.degree. C. Within this temperature range, secondary reactions causing coke deposits are generally no longer encountered. This cooling causes partial condensation of the reaction gases. Carbon black particles entrained by the gas stream from the reaction furnace are suspended in the cooled liquid product and can be retained by filtration. For the further processing of the reaction product mixture, the individual components, i.e. hydrogen chloride, vinyl chloride and unconverted 1,2-dichloroethane are isolated in the sequence of their boiling points from the higher-boiling substances, and the 1,2-dichloroethane is recycled to the reaction furnace.
According to a publication which appeared in "Hydrocarbon Processing", November, 1975, pages 214/215 the commercial processes of the two principal licensors make use of the direct cooling of the hot reaction gases by means of cooled liquid product in a multitude of industrial plants.
The disadvantage of the method of direct cooling which has been practiced for more than 20 years is the high energy demand for pumping the recycle flow of reaction product and the complete loss of the heat supplied to the reaction furnace.
It is also known to separate carbon black from the effluent reaction gases by means of a cyclone with subsequent cooling by air; however, as compared to cooling with liquid 1,2-dichloroethane, this method requires equipment of considerably greater volume for high throughput rates without eliminating the undesirable heat losses.
The description also covers the cooling of the reaction gases with water in one stage. In view of the low temperature level to which the gases must be cooled to undergo partial condensation before being fed to the first column for separating the hydrogen chloride, this method is largely inadequate to utilize the heat content of the reaction gases. Moreover, the description contains no information on the actual cooling rate of the reaction gases nor on the service time of the coolers.
Another disadvantage of direct cooling is the volume of equipment and machinery required. For lowering the temperature of the hot reaction gases from approximately 530.degree. C. to approximately 80.degree. C., for example, the quantity of recycle fluid is about 17 times the quantity of liquid product. The heat absorbed by the recycle fluid is dissipated on the reaction product side for the temperature range from about 80.degree. C. to 40.degree. C. and on the cooling water side at a temperature of about 25.degree. to 35.degree. C. Cooling of the liquid recycle fluid must be maintained and secured at any rate to avoid a dangerous temperature rise in the quench tower. The safety system provided for this purpose comprises a temperature interlock for the reaction furnace actuated by a temperature sensing element at the quench tower outlet, and of an emergency power set and a steam turbine for driving at least one fluid recycle pump. At the time of shutting down the plant, this is the only possibility to dissipate without risk, through vaporizing 1,2-dichloroethane, the heat stored in the furnace bricklining.
The reaction furnace is generally placed in the immediate vicinity of the quench tower. The connecting line, for example, has a length of only 1 to 2 m. The admission of reaction gas into a quench tower at a temperature of 530.degree. C. is bound to cause excessive stresses in the material unless injection of cold liquid recycle product from the top is safeguarded. Under extreme conditions explosive fluid may escape through leaks in the quench tower in the immediate vicinity of the furnace burners.